1. Technical Field
The present invention relates generally to variable frequency driven motors and in particular to brake systems within variable frequency driven motors. Still more particularly, the present invention relates to a method and apparatus for responding to brake failure in variable frequency driven motors.
2. Description of the Related Art
Variable frequency driven motors are utilized for a variety ot lifting mechanisms, from overhead cranes and hoists to elevators. Typically, when a load supported by a variable frequency driven motor is to be held in suspension for a significant period of time, an external brake is set and the motor is disengaged from supporting the load. This allows savings in the power required to supporting the load using the motor and also saves the motor from unnecessary wear.
When a brake is employed to hold a load in suspension, various events may result in release of the load, including brake failure and inadvertent release. The brake may fail as a result of a broken spring or a failed brake shoe. Inadvertent release of the load may result from accidental closing of contacts during maintenance. In either case, the effect of dropping the load may be disastrous.
It would be desirable, therefore, to provide a mechanism for preventing a load elevated by a variable frequency driven motor and supported by a brake from dropping n the event of brake failure. It would further be advantageous if the mechanism could be incorporated into existing commercial embodiments of variable frequency driven motors without the introduction of additional components.